1. Field of the Invention
This invention relates to a switching arrangement for controlling the polarisation of multi-element antenna arrays and more particularly to antenna arrays used in Radio Frequency Identification Systems.
2. Description of Related Art
Radio Frequency Identification (RFID) technology utilizes a tag transponder, which may be associated with/attached to an object, and a reader generating an interrogation signal to read and identify the RFID tag(s) within range of the interrogation signal. RFID technologies are broadly categorized using “active” tags with a local power source enabling longer read ranges and/or the communication of greater amounts of data, and unpowered “passive tags” typically transmitting only a unique RFID tag identifier in response to an interrogation signal.
A typical RFID tag includes an electronic circuit that may be in the form of an integrated circuit or silicon chip, whereby the circuit stores and communicates identification data to the reader. In addition to the chip, the tag includes some form of antenna that is electrically connected to the chip. Active tags incorporate an antenna which communicates with the reader from the tag's own power source. For passive tags, the antenna acts as a transducer to convert radio frequency (RF) energy originating from the reader to electrical power, whereby the chip becomes energized and performs the communication function with the reader via backscatter modulation. Alternatively, a passive tag may be coupled to an energized circuit, responding with dynamic data from the energized circuit, such as environmental/status data such as temperature, humidity and/or battery condition.
An RFID communication system may include scanning interrogation beam technologies to focus the interrogation signal upon a designated location within a target space, thus identifying with greater sensitivity/accuracy the presence, location and/or direction of movement of an individual RFID Tag within a three dimensional target area. For example, International Patent Application publication number WO 2009/035723, titled “Radio Frequency Signal Acquisition and Source Location System” filed Mar. 30, 2008 by Bloy et al, and International Patent Application publication number WO2009/034526, titled “Steerable Phase Array Antenna RFID Tag Locater and Tracking System”, filed Sep. 9, 2008 by Bloy, both applications commonly owned with the present application and hereby incorporated by reference in their entirety, describe systems of cooperating steerable phased array antennas performing beam scans of a target area, via an electronic beam steering circuit such as an array of phase shifters coupled to a corresponding array of antenna elements of a panel antenna, from which the presence and location of individual RFID tags is derived by logical processing of historical signal data obtained from prior scans of the target area.
In environments where a large number of RFID tags are present, the ability of the reader to read each of the RFID tags, the read rate, may be significantly degraded.
The degradation may be generated by interference from other RFID tags and/or the RFID tags may block or partially block one another along a signal path to the antenna generating the interrogation signal.
The orientation of the RFID tag and/or tag antenna with respect to the interrogation signal path will determine the signal level received by the RFID tag and/or any response signal generated by the RFID tag exposed to the interrogation signal. For example, an RFID tag oriented in a plane normal to the interrogation signal path will provide a stronger signal response than an RFID tag oriented in a plane parallel, an edge view, to the interrogation signal path.
Interrogation signals may be launched from the reader antenna with a desired electric field plane polarization, such as vertical, horizontal or circular polarization. For vertical and horizontal polarization, the electric field plane is oriented either vertically or horizontally. For circular polarization, the electric field plane is rotated during modulation, for example rotating in a circle making one complete revolution during one period of the wave.
Linear polarity interrogation signals, vertical or horizontal, when aligned with the antenna orientation of the RFID tag, may provide improved communications performance compared to circular polarity interrogations signals. However, communications performance is significantly degraded in linear polarization configurations, if the signal/antenna alignment is not optimal. Circular polarization interrogation signals provide reduced communications performance but enable communications with RFID antennas in a much larger range of RFID tag orientations. However, where RFID tags are closely spaced, circular polarisation interrogation signals may experience significant communications performance degradation, thereby reducing the amount of energy available for each of the closely spaced tags, reducing the minimum operating distance between a reader and the plurality of tags and/or requiring increased incident/transmit power from the reader. It may not always be possible to increase reader transmit power because of radio regulations and decreasing the distance between the reader and the plurality of tags may not be possible because of the use case or physical environment.
U.S. Pat. No. 6,367,697 “Reader Arrangement for an Electronic Identification System having a Plurality of Reader Heads for Energizing Transponders” by Turner et al, teaches an reader arrangement in which multiple antennas and/or multiple element antenna arrays may alternatively utilized during RFID tag communication to improve communications performance with the diversity of antennas associated with RFID tags. Although U.S. Pat. No. 6,367,697 teaches switching and phase delay, it does not disclose or suggest a means to change and/or dynamically switch the polarisation of the antenna array and simultaneously accommodate the phasing and splitting circuits required.
Therefore, it is an object of the invention to provide antenna switching arrangement(s) and method(s) that overcome deficiencies in the prior art.